Magnetic deflection sweep circuit



Dec. 27, 1955 VARELA 2,728,876

MAGNETIC DEFLECTION SWEEP CIRCUIT Filed Feb. 21, 1946 CURRENT TIME ARTHUR A. VARELA au'omuu United States Patent MAGNETIC DEFLECTION SWEEP CIRCUIT Arthur A. V-arela, Washington, D. C. Application February 21, 1946, Serial No. 649,442

Claims. (Cl. 315-27) (Granted under Title 35, U. S. Code (1952), see. 266) This invention relates to electronic circuits and more particularly to magnetic electron-beam deflecting circuits of the type employed in electronic television systems, cathode-ray Oscilloscopes, and cathode-ray tubes in general, wherein the electron beam is deflected by a varying magnetic field that builds up slowly at a uniform rate and decays suddenly, or vice versa.

As is well known, the electron beam is moved across the cathode-ray tube, usually in a horizontal direction, at a constant rate of speed, the magnitude of the movement being proportional to the elapsed time from the beginning of that particular movement. This movement is com= monly called the sweep. Such uniform deflection is obtained by permitting the beam of electrons in the cathoderay tube to traverse a region in which either an electric or magnetic field is caused to build up or to decay at a uniform rate. Although electrostatic deflection is generally more conserving of energy than the magnetic method, unusually high voltages are required to obtain sufficient deflection and adequate focusing of the electron beam. Hence, because of these and other reasons, the magnetic deflection type of tube is commonly preferred.

A uniformly varying magnetic field may be obtained by establishing in a coil a current having a sawtooth wave form. Several considerations are of primary importance in the design of a magnetic deflecting system with an output current wave form of the proper shape. One involves the production of a linear sweep current in the deflection coils despite the presence of undesirable and fortuitous circuit elements such as the distributed capacitance of the deflecting coil and of the circuit wiring. A second consideration is the use of electronic tubes which do not have as linear a plate current characteristic as is desirable. Since a uniformly varyingsawtooth voltage can rather easily be obtained from known circuits, the invention primarily concerns'itself with obtaining a linear current sawtooth wave form from a circuit with a linear sawtooth voltage input.

' Accordingly, one object of the invention is to provide a means for producing a substantially linear sawtooth current in the deflecting coil of a magnetic deflecting system.

Another object of the invention is to provide a high efliciency magnetic deflection system of the type actuated by a sawtooth voltage input wave, the circuit having compensation for the presence of fortuitous circuit elements and non-linear electronic tube characteristics.

The nature of the present invention together with other objects and features thereof will appear more fully in the following description, reference being made to the appended drawing, in which:

Fig. 1 shows'diagrammatically the circuit of a fundamental magnetic deflection sweep system;

Fig. 2 is a diagrammatic illustration of an embodiment of the invention;

Fig. 3 is a series of curves showing current-time relationships.

' 'By way of introduction and before considering the methods and apparatus of the invention, it will be well to consider the operation of the fundamental magnetic deflection sweep circuit which is excited or triggered into operation by a sawtooth or trapezoidal voltage input. Referring now to Fig. 1, which illustrates diagrammatically the circult of such a sweep system, a deflection coil 1 is connected in the plate circuit of a power or driver tube 4-, plate voltage being supplied to the circuit by the battery 6 or other direct current source, and the driver tube 4 being biased by the cathode biasing resistor 5. The fortuitous circuit elements appearing in the deflection coil are shown dotted and are represented by the inherent resistance of the coil 3 and the distributed capacitance 2.

The wave shape of the ideal deflection current is a sawtooth 8. T o produce this sawtooth current through a pure inductance, a rectangular voltage wave is required due to the charging effect of the inductance. To maintain a sawtooth current in the inherent resistance of the coil, and also in any other resistance appearing in the driver tube plate circuit, a voltage of sawtooth form is required. Hence, the voltage across the deflection coil 1 in the plate circuit of the driver tube 4 must consist of a rectangular voltage plus a sawtooth voltage, or, in other words, a trapezoidal voltage. This voltage consists of an initial jump proportional to the inductance in the circuit and the rate of change of the current and an ensuing linear rise of voltage at a rate proportional to the total resistance of the plate circuit and the rate of change of the current. Therefore, the voltage wave applied to the grid of the driver tube must also be of trapezoidal shape and in the same sense as the voltage rise, or drop, across the plate circuit load elements, but of a smaller magnitude depending on the amplification of the driver tube.

There are a number of ways of generating the trapezoidal voltage for the input of the driver tube, but none of these are shown since they do not form any part of the invention and are well known in the art. The current in the deflection coil will not fall immediately to zero at the end of the sweep, but will fall in an exponential manner at a rate depending on the amount of resistance and inductance in the circuit. This decay will cause no trouble since the circuit elements are so chosen that the current will fall to zero before the start of the next sweep, this being the returner liyback time of the cathode-ray trace.

The deflection coil has distributed capacitance 2 associated with it, as has the circuit wiring. In effect, this capacitance will be in parallel with the deflection coil inductance. Since the voltage across a condenser in a series resistance-capacitance circuit cannotbe changed suddenly, the distributed capacitance will reduce the steepness of the voltage Wave front. This results in a reduced initial jump of the trapezoidal voltage, causing the wave form to approach the shape of a sawtooth. The application of a sawtooth voltage to a series resistanceinductance circuit results in a current that increases slowly at first and then gradually becomes linear with time. This characteristic is shown graphically by curve B of Fig. 3. The desired current-time characteristic is shown by curve A of the same figure. Therefore, it is observed that the distributed capacitance results in a non-linear deflection current, causing the cathode-ray scan to be.

. the effect of the distributed capacitance of the deflection coil and circuit wiring by varying the gain of the sweep zoidal wave.

system at the beginning of the sweep. To control these actions, a two-stage voltage amplifier is used, feeding into the driver stage. This embodiment is shown schematically in Fig. 2.

Referring now in particular to Fig. 2, the circuit of the invention comprises three vacuum tube amplifiers 9, 12, and 16 arranged in cascade. The final amplifier tube 16 serves as the driver stage for the magnetic deflection coils. A trapezoidal voltage wave of a frequency equal to the desired sweep frequency is fed into the grid of the first amplifier tube 9. This tube may now be assumed to conduct and amplify in the conventional manner, a change of current flow through the tube governed by the changing grid voltage causing an amplified but inverted voltage change to appear across the load resistor 10. This inverted voltage is fed through the coupling condenser 11 to the grid of the second amplifier tube 12. The grid leak resistor 13 for this tube is connected between the grid and the cathode of the tube, the cathode being connected to the negative side of the power supply, designated as B, and also being by-passed to ground potential through the bypass condenser 14. The second amplifier tube 12 is so biased and connected that his conducting continuously. The grid variation on this tube causes the current flow through the tube to vary so that the voltage variation across the load resistor 15 is of the same sense or polarity as the trapezoidal input voltage 7. Since B is of such a value that the plate of the second amplifier 12 rarely swings into the positive voltage values, the amplified trapezoidal wave form may be fed directly to the grid of the driver tube 16. The plate load of this tube consists of the deflection coils 1', here shown as two coils which are wound on opposite sides of the circular iron yoke in order to give a uniform magnetic field across the cathode-ray tube, and the shunting resistor 17 used to limit the current through the deflection coils. The screen grid receives voltage from the screen dropping resistor 18, which, with resistors 19 and 20, forms a voltage divider for biasing the amplifier tube 16.

As thus far described the wave form of the current passing through the deflection coils 1 in the circuit of Fig. 2 has a non-linear rise or a curving rise corresponding to that shown in curve B of Fig. 3. To compensate for the non-linearity caused by the characteristics of the driver tube 16, inverse feedback as taught by the invention is used. When the grid voltage of the driver tube 16 rises in accordance with the positive trapezoidal wave impressed on it, current flow through the tube causes a similar voltage to appear across the cathode resistor 20.

This resistor, not being by-passed, provides a certain amount of feedback and hence linearity to the operation of tube 16. This voltage 24 is fed back through a feedback resistor 21 to the cathode of the first amplifier tube 9 as shown schematically in Fig. 2. Thus this positive i voltage Wave form in eifect reduces the amplitude of the exciting wave form 7, providing inverse feedback, A desirable characteristic of inverse feedback is that it reduces non-linear distortion produced in the amplifier by controlling the overall amplification or gain of the amplifier.

There is, however, the effect caused by the charging of the distributed capacitances of the deflection coils and circuit wiring which must be compensated for if linearity is to be obtained. It is well known in the art that the steepness of a wave front depends upon the number of harmonics and higher frequencies appearing in the wave. Since the feedback voltage 24 has alrather sharp jump at the beginning of the Wave form, a large number of thesehighcr-frequency components comprise the trape- When the feedback wave reaches the cathode of the amplifier tube 9, the circuit comprising resistor 22 and condenser 23 appears as a short circuit to ground to the higher frequencies in the wave. The resistor 22 and the condenser 23 have a very small charging time constant; hence the voltage is decreased at the beginning of the feedback wave due to the lay-passed high frequencies while the condenser 23 is charging. Therefore, at the beginning of the trapezoidal input wave less voltage is fed back into the first amplifier stage, and the gain of that stage and, hence, of the system is a maximum. As the wave continues the condenser 23 becomes charged and less of the high frequency components are shunted to ground. As the voltage on the cathode of the first amplifier rises, the gain of the stage decreases until the nor mal gain is again reached. Increasing the amplification in the circuit has the effect of raising the current wave form, eliminating the delay at the start of the current sawtooth and making the current wave form more linear.

The circuit constants of a generator such as illustrated in Fig. 2 which have proved highly successful in actual practice for amplifying sweep voltages and producing a linear current sweep at very high repetition rates are as follow:

Resistor 10 ohms 27,000 Resistor 13 do 680,000 Resistor 15 do 270,000 Resistor 18 do 8,600 Resistor 19 do 22,000 Resistor 20 do 330 Resistor 21 do 2,700 Resistor 22 do 220 Condenser 11 microfarads 0.05 Condenser 14 do 1.0 Condenser 23 do 0.0015

Tubes 9 and 12 are triodes and may be in separate envelopes such as the 615 tubes, or they may be combined into a dual-triode such as the 6SN7 tube. Tube 16 is of the 6L6 beam power type. +B is 300 volts direct current and B is approximately minus eighty (-80) volts direct current.

Thus there has been described an amplifier and sweep generator for magnetic deflection sweep currents which is triggered by a trapezoidal input voltage wave form and which produces a linear deflection current of sawtooth wave form. The circuit of this invention is primarily for use with sweeps of high repetition rate, but by careful choice of circuit constants satisfactory operation may be obtained at other frequencies. It will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the intent of the invention as set forth in the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. An electronic circuit for producing the flow of a linear current in the deflection coils of a magnetic deflection circuit in response to a trapezoidal input voltage wave, comprising a vacuum tube amplifying means having an anode, grid and cathode elements, means connecting said trapezoidal input wave to said grid element, means for connecting the output of said amplifying means from said anode to said deflecting coils, and a degenerative feedback connection between said last named means and said cathode including a resistive-capacitive circuit connected between said cathode and ground operative to attenuate the higher frequencies in the feedback wave during at least the beginning of the waveform of said feedback wave.

2. An electronic circuit for producing the flow of a linear current in the deflection coils of a magentic deflection circuit in response to a trapezoidal input voltage wave, comprising a first vacuum tube amplifying circuit means having an anode, grid and cathode elements, means connecting said trapezoidal input wave to said grid element, a second vacuum tube driver circuit having an anode, cathode and at least one grid element, means connecting the output of said first vacuum tube at said anode to the grid of said second vacuum tube, said deflecting coils connected as a load in the anod: circuit of said driver tube, and a degenerative feedback connection, between the cathodes of said first and second vacuum tubes, including a frequency sensitive network connecting the cathode of said amplifying vacuum tube means and ground operative to attenuate the higher frequencies in the feedback wave during at least the beginning of the waveform of said feedback wave.

3. An electronic circuit for producing the flow of a linear current in the deflection coils of a magnetic deflection circuit in response to a trapezoidal input voltage Wave, comprising a first vacuum tube amplifying circuit means having an anode, grid and cathode elements, means connecting said trapezoidal input wave to said grid element, a second vacuum tube driver circuit means also having an anode, cathode and at least one grid element, means connecting the output of said first vacuum tube at said anode to the grid element of said second vacuum tube, said deflecting coils connected as a load in the anode circuit of said second vacuum tube, and a degenerative feedback circuit connecting the cathodes of said first and second vacuum tubes including a series resistive element and an unbypassed resistive element connected from the cathode of said second vacuum tube to ground and a resistive-capacitive circuit serially connected between the cathode of said first vacuum tube and ground, said resistive-capacitive circuit operative to attenuate the higher frequencies in the feedback wave during at least the beginning of the waveform of said feedback wave.

4. An electronic circuit for producing the flow of a linear current in the deflection coils of a magnetic deflection circuit in response to a trapezoidal input voltage wave, comprising a first vacuum tube amplifying circuit means having an anode, cathode and grid elements, means connecting said trapezoidal input wave to said grid element, a second vacuum tube amplifying means having an anode, cathode and grid elements, means connecting the anode of said first vacuum tube to the grid of said second vacuum tube, a third vacuum tube circuit operative as a driver stage having an anode, cathode and at least one grid electrode, means connecting the anode of said second vacuum tube to the grid of said driver vacuum tube, said deflecting coils connected as a load in the anode circuit of said driver tube, and a degenerative feedback connection between the cathodes of said first amplifying tube and said driver tube and including a frequency sensitive network connecting the cathode of said first amplifying means and ground operative to attenuate the higher frequencies in the feedback wave during at least the beginning of the wave form of said feedback wave.

5. An electronic circuit for producing the flow of a linear current in the deflection coils of a magnetic deflection circuit in response to a trapezoidal input voltage wave, comprising a first vacuum tube amplifying circuit means having an anode, cathode and grid elements, means connecting said trapezoidal input wave to said grid element, a second vacuum tube amplifying circuit means having an anode, cathode and grid elements, means connecting the anode of said first vacuum tube to the grid of said second vacuum tube, a third vacuum tube circuit means operative as a driver stage having an anode, cathode and at least one grid element, means connecting the anode of said second vacuum tube to the grid of said driver tube, said deflecting coils connected as a load in the anode circuit of said driver tube, and a degenerative feedback circuit connection between the cathodes of said first and third vacuum tubes including a series resistive element and an unbypassed resistive element connected from the cathode of said third vacuum tube to ground and a resistivecapacitive network serially connected between the cathode of said first vacuum tube and ground, said resistive-capacitive circuit operative to attenuate the higher frequencies in the feedback wave during at least the beginning of the waveform of said feedback wave.

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